Word: loops
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They don't come expecting exotic sights. Once you've made one loop around the small island where the ducks live, there's nothing too exciting to see. Instead, the "swanboat experience" is passive. Sit on the park benches bolted to these flat-bottomed tubs, let the sun heat you shoulder-blades, and listen to the subdued whir of the propellers...
...sequence of human errors and mechanical failures began two weeks before the mishap. As part of a test, valves in three auxiliary pumps in the plant's secondary loop, which carries superheated water to the turbines that drive the electrical generators, were shut down. Incredibly-and in violation of NRC regulations-they were not reopened before the plant was put back into operation...
What triggered the accident was the failure of a pump in the secondary loop that transports hot water from the reactor. When this happened, the auxiliary pumps switched on as they were supposed to do. But, with their valves shut, they could not pump water. Their failure backed up water in the secondary loop and sent pressure inside the reactor soaring. This pressure rise, in turn, caused a relief valve to pop open. It stuck. Pressure then dropped so rapidly that the emergency core cooling system, designed to keep the core from overheating, was automatically activated. That started a reactor...
...calculated levels. The chain reaction begins, converting mass into energy and producing great quantities of heat in the process. That raises the temperature of the water surrounding the core to nearly 600° F. Under high pressure this water is carried off by the cooling system's primary loop, a complex system of pipes, to a heat exchanger called a steam generator. The heat is transferred from the radioactive water of the primary loop to the uncontaminated water of the separate secondary loop, where the water is quickly heated to steam that drives a turboelectric generator, which in turn...
...known as a "meltdown," in which the fuel melts through the floor of the containment building into the ground and possibly erupts in a geyser of steam and debris upon hitting the ground water, releasing a radioactive cloud into the air. As the final precaution, the reactor and primary loop are shielded by a thick concrete containment dome, which should prevent the venting of any radioactivity into the atmosphere-as long as a meltdown does not occur and if there are no other mishaps or blunders. Obviously, at Three Mile Island, these fail-safe systems somehow failed...